Suspension bridge



Nov. 11 1924. 1,514,932

H. D. ROBINSON ET AL SUSPENSION BRIDGE F iled Feb. 15.

2 Spats-Sheet 1' 4 W M 7 H 2 R n Y m a M Mn m o whvm .n 8 5 I V a 4 H0 w m I m Nov. 11 1924. 7 1,514,932

. H. D. ROBINSON ET AL SUSPENSION BRIDGE Filed Feb. 15. 1923 2 Sheets-Sheet 2 34 r 2a 29 r 26 34 33 3.3 p- I] 27 /VV;! ,!!vvYYYv 33 34 2 f? 6 II 2 27 2 33 v IIVKEA/ 70R; f/o/fon D. Rabmson ATTORNEY Patented Nov. 11, 1924.

Q Nil Fill S TAT HOLTON D. ROBINSON AND DAVID B. STEINMAN, OF NET/V YORK, N. Y.

SUSPENSION BRIDGE.

Application filed February 15, 1923. Seriafl No. 619,090.

To all whom it may concern.

Be it known that we, HoLToN D. Room SON and DAVID B. STEINMAN, citizens of the United States, and residents of the city of New York, in the county of New York and State of New York, have invented certain new and useful Improvements in Suspension Bridges, of which the following is a specification.

This invention relates to improvements in suspension bridges and the design thereof, and involves a revised outline and a new combination of the component structural elements. Y

One of the objects of this invention is the provision of a suspension bridge consisting of a minimum number of parts, and which may be more easily erected, is more economical, islighter and is more efficient than the types heretoforeemployed.

Another object of this invention is to provide, for suspensions bridges, stiffening trusses or girders of variable depth conforming scientifically to the stress variations along the span, in order to secure maximum uniformity of sections.

Another object of this invention is the provision of a suspension bridge with stiffening trusses having outlines conforming scientifically to the variation in bending moments along the span, in order to se cure maximum economy of design.

Another object of this invention is to provide a suspension bridge having stiffening trusses with increased depth and stiffness at such points as are subject to critical defied tions, in order to secure great rigidity against vertical deflection.

the accompanying Another object of this invention is to pro vide a suspension bridge in which all longitudinal forces, such as traction ano braking, may be taken up by the cables or chains.

Another object of this invention is to provide a form of braced suspension bridge in which the foregoing advantages are combined with maximum facility and simplicity of erection.

For the attainment of the aforesaid and other objects as will hereinafter appear. we employ the arrangements and combinations of parts shown in diagrammatic outline in drawings, in which,

Fig. 1 is a diagrammatic elevation of a singl l span suspension. bridge embodyi' the principst elements of our inven Fig. 2 is a graphshow the variationalong the span of governing bending moments in a single-span suspension bridge. Fig. 3 is a diagrammatic elevation of a modified form of three span suspension bridge, the left half of which is similar to the right half of Fig. 4 and the right half of which is similar to the let half of Fig. l. is a diagrammatic elevation of a single-span. suspension bridge, showing in the left half thereof a modified form of truss similar to that in Fig. l, and in the right half thereof, another modified form of stiffening truss Fig. 5 is a diagrammatic elevation of another form of suspension bridge in which the chain forms part of the bottom chord and acts as a member of the stiffening truss. Fig. 6 is a diagrammatic elevation of a snspension bridge similar to the right half of Fig. 4, embodying the principal elements of our invention in each of three spans. Fig. 7 is a graph showing the variation of governing bending moments in the main span and in the side spans of a three span suspension bridge. Fig 8 is an elevation, similar to Fig. 6 of another form of three span suspension bridge in which the cable or chain is connected to the side span trusses to take up the horizontal tension, whereby the bridge can be made self anchoring.

A suspension bridge designed by us and embodying the principalelements of our invention is now under construction at Florianopolis, Brazil, and has a main span of approximately 1114. feet. An outline illustrating the aforesaid practical. embodiment of our invention is shownin Fig. 1.

Suspension bridges heretofore generally used consist of carrying members such as cables or chains, and stiffening trusses separate from said members and suspended therefrom. Said stiffening trusses each have substantially parallel top and bottom chords, giving uniform depth throughout the truss and are usually hung from the carrying members by suspenders arranged at intervals throughout the span.

In our improved construction, we secure a scientific variation in the depth of the stiffening truss offering valuable advantages in strength, rigidity and economy over the types of bridges above mentioned, as will be hereinafter more fully explained. In addition, we prefer to connect the chains directly to the stiffening trusses a are thereby enabled eliminate the central portion of Fig.

in the side-span, are thereby entirely dis pensed with.

In the various forms of our improved sign, the usual towers 10 and chains 11 provided. stiffening trusses as 12 to 2 elusive, are suspended from portions ot chain as by means of the usual suspei l 26. Each of. said stiflening trusses 3r. ably has a substantially horizontal l: chord 25, and is preferably arranged to i or join the tower at a point intermediate oi the ends of said tower.

In Fig. 1, the central portion of: the chain 11 forms the central part 27 of the top chord of the stiffening truss 12, the points of juncture 28 and 29 of said chain and truss be ing at or near the quarter points oi the span. Similarly, the lowermost section 2?" of said chain 11 forms part of the top chord of the trusses 14, 15, 1?, 18 and 9 as illustrated in the figures, and is preterably joined at or near the quarter 28 and 29 of'the span in the same manner as; that shown in Fig. 1.

Details of said joints a" 28 and 29, and of the trusses, towers, chains, and other parts are not shown, since said parts may be de signed by one skilled in the art, and the specific structure forms no part of this in vention. It is to be understood, however, 1

that port-ions of the chain may be uti-.. en

to replace certain of the truss members other than the top chord, as is illustrated in Fig. 5, in which the web members 80 and 31, and a portion 32 of the bottom chord are so replaced. In a multiple span bridge, certain members of the side sgan stiffening trusses may similarly be replaced by the outer portion 33 of the baclistays 36, which may be joined to the trusses it 20, 21, 2:2, 23 and 2st at or near the midpzfnt thereof. (Figs. 6 3 and 4-).

The chains 11 hang in a curve which is approximately a parabola ora catenary, and are suitably suspended from the towers 10. Continua-tions of the chains above mentioned, or other chains, constitute the backstays 36, which extend from the tops of the towers to the anchorages 3'7. Said baclrstays are commonly referred to as straight.

but are actually slightly curved as catena' ries. The principal carrying members it and the backstays 36 may be chains and made ofeyebars, flats, members having the cross sectional shape of any of the usual structural shapes, or other suitable elements, or cables and made of wires, wire ropes,

rope strands, or other suitable and comparatively flexible elements. It is to be understood that, in this specification, the words chain and cable may replace each other, the functions of both being identical.

In the usual forms of suspension bridges, the cables or chains cannot take up any of the horizontal or longitudinal forces acting on the stiffening trusses, such as traction or braking, due to the fact that no direct connection between the chains and trusses is provided. lVe provide such a direct connectiat ZSand 29 well as at other points i the oit' thespan; in fact, that portion oi chain included between the connecting pl 28 and 29 is an integral portion and torn'i: certain members of the truss. Consequently, the longitudinal forces are transferred to the chain, giving certain advantages oi rigidity, economy and simplicity in the design.

T he principal source of economy is in the elimination of a large portion of the top or other chords of the stiffening trusses between the points 28 and This saving is really an elimination of a glaringly unscientific feature in the conventional design, in that a heavy member in compression, namely the central portion of the top chord, is ordinarily allowed to remain substantially parallel and in close juxtaposition to a more powerful neighbor member, namely the cable or chain, in tension.

By combining the aforesaid members into a single member, there is accomplished a subtraction of stresses, instead of an addition of sections'some taking tensile and some taking compressive stress. That is, where the chain is used to replace part oi? the top chord, the compressive stress in said top chord of the stiffening truss is balanced .nst and lessens the tension in the cable, and the top chord section usually provided to take said compression is dispensed with. This elimination of a large and heavy por tion of the top chord permits a saving of one-fifth or more of the metal in the still ening truss. Similarly, a portion of the bottom chord between. points 38 and 39, or certain web members as 30 and 31 may be eliminated as shown in Fig. 5, with consequent saving of material and weight.

Another important source of economy is in the new configuration of the stiffening truss. departing from the conventional form of approximately constant depth.

It is a demonstrated principle in structural design that the economical depth at any section of a truss or girder is a direct function ot the governing bending moment Cir governing bending moments along the span.

' distances the bending moment at the corresponding point of the span It will be noted that the bending moments are greatest in the vicinity of the quarter points of the span.

The above stated principle of economical design would therefore demand that the stiff ening truss of a suspension bridge should conform as closely as is mechanically possible to the profile indicated in Fig. 2, whereby said truss would have a maximum depth at or near the quarter points of the span.

Instead of a truss, any other kind of girder may be used to stiffen the suspension bridge, and the words girder and truss are therefore used interchangeably in this specification. 7

It will be noted that the truss outline shown in Fig. 1 provides maximum depth near the quarter points of the span with reduced depth at mid-span and near the ends of the span.

In addition to the saving in metal, this variation in truss depth yields greater uniformity of chord sections, thereby further contributing to economy in design and in details.

As a result of the above described economics and of other detail savings in the design shown in Fig. 1, there is a total saving of approximately a third of the total weight of the stiffening truss. This reduction of loadresults in a corresponding reduction in the stresses in the chains, with a resulting material economy in their sections, and also in a reduction of the stresses in the susponders. The lightening of the trusses, suspenders and chains results in. a material reduction in the total tension in the chains, which in turn permits a considerable saving in the towers and in the anchorages. Each of the above mentioned reductions of stress represents an increase in strength of the structure or a saving in material, or both.

These advantages of increased strength, and economy are further supplemented by a very important increase in rigidity, resulting primarily from the improved truss profile shown in Fig. 1. A truss or girder having a profile conforming to the govern ing bending moments will naturally yield.

greater rigidity than one with parallel chords or flanges. Moreover, the critical deflections of a suspension bridge are peculiar in that they are produced by a live load. covering about half of the span. Under this condition, the loaded half of the span defleets downwardly and the unloaded half curves convex upwardly, with resulting objectionable deflection gradients. This characteristic deflection is most effectively resisted by increasing the stiffness near the quarter points of the span. Our improved. design possesses great stiffness near the quarter points where itis most needed, since the stiffness is proportional substantially to the square of the depth. The structures shown are about twice as deep at the quarter points as is the conventional design with parallel chords; as a result, the critical defleotions are reduced to about a quarter of their usual magnitude.

Actual computations comparing the design shown in Fig. 1 with the conventional suspension bridge design having an independentstiffening truss with parallel chords, show that the stiffening truss in our improved design weighs only two-thirds as much as the conventional type, but is over four times as stiff against deflections.

The direct connection between the chain and the stiffening truss, not shown in detail, is another contribution to the rigidity of, our design as is also the positioning of the heavy chain sections so that said chain sections contribute to the moment of inertia of the truss.

Various modifications, of outline of the truss can easily be made to suit individual preferences. Fig. 4: shows two of the possible variations fromthe' outline shown in Fig. 1. The point of juncture of'the chain and chord may be varied in position and in arrangement, the outer portions of the top chord may be made straight, broken, or curved. the web systems of the trusses may be modified, the end panels of the trusses may be given various arrangements as indicated in the figures, and other changes may be made without affecting the basic principles of our invention. An application of said principles to multiple span bridges is indicated in Figs. 3, 6, and 8.

In the three span suspension bridge shown in Fig. 6, the main span is substantially like that shown in Fig. 1. The backstays 36 (Fig; 6) are curved instead of straight, and from them are suspended side spans of the stiffening truss. The junctions between the chain and top chord in the side spans are made at points 84., preferably at or near the mid-points ofsaid spans. Beyond "said points 34, the chains 36 replace the top chordsof the side spans. It will be noted that thetrusses in each ofthe three spans are thereby providedwith profiles correQ spending substantially to the profiles of the respective moment graphs 44, 4:5 and ib, Fig T. Since the governing bending inoments in the side spans are greatest at mid span, the economy and rigidity of the side span trusses are promoted by giving them a and the profile varied to. conform to any resuiting changes in the moment graphs.

Fig. 8 illustrates a bridge in which the principles above outlined are utilized to provide a convenient method of self anchorage for a suspension bridge, dispensing with the external anchorages ordinarily required and which are usually expensive items in the cost of such structures. In said figure, the horizontal tension of the cables or chains in the bridge is taken up by the stiffening trusses instead of being carriedoutside of the structure into masonry or rock. The resulting horizontal reaction of the side span trusses must then be providedfor in the design and details of the trusses and towers, as will be readily understood.

Other combinations of chains and stiffening trusses, whereby portions of said chains are utilized to replace portions of said trusses, may be made in accordance with the underlying principles of our invention. Figs. 1, 3, 4, G, and 8, show only substitutions in the top chord, but substitutions in the bottom chord or in the Web members as indicated in Fig. 5 may also be made if desired in order to SQClll-BllllQ corresponding advantages, and We therefore do not wish to limit ourselves to the specific structures illustrated herewith.

lVe claim:

1. In a suspension bridge, a partly combined chain and stiffening truss comprising a chain element and a stiffening truss element, said stiffening truss element having an upper chord wherein the mid-portion is the mid-portion of the chain element, and wherein the remainder of said upper chord is designed for compression.

2. In a, suspension bridge, towers, a stiif en-ing truss arranged intermediate of the ends of said towers, a bottom chord substantially straight throughout its entire length on said truss, and a chain having its lowermost portion replacing and acting as part of the upper chord of said truss and arranged u'ith the remaining part of said chain in spaced relation, and flexibly connected to said truss.

In a suspension bridge, a pair of towers, a stiffening truss arranged in the span between, and intermediate the ends of said towers, a substantially horizontal bottom chord in said truss, an upper chord in said truss having its outermost portions designed for compression, a chain suspended from said towers, a truss section of the mid-portion of said chain forming the mid-portion of said upper chord, and suspenders designed ior tension flexibly connecting the remainder of said chain to the remainder of said truss.

at. In a suspension bridge, a stiffening truss comprising a bottom chord straight for substantially the full length of the span thereof, and a top chord having its outermost portions designed for compression spaced at any point in the span from said bottom chord a variable distance, substantially in direct ratio to the bending moment in the truss at said point. i

5. In a suspension bridge, a stiffening truss having a depth at any point variable in proportion to the bending moment at that point, a chain, its outermost portions substantially independent of said truss and having its mid-portion forming part of said truss, and flexible means designed for tension for suspending said truss from said chain at spaced intervals of the span where said chain forms no part of said truss.

6. In asuspension bridge, a chain, a stiffening truss in spaced relation to said chain for the greater part of its span, comprising a bottom chord substantially straight for the entire span thereof, and a top chord, spaced at any point in the span from said bottom chord, a distance proportional to the bending moment at said point, and flexible means :torsuspendin said truss from said chain.

7. In a suspension bridge, a stiffening truss in which the depth at any point in the span is proportional to the bending moment at said point, and a chain so connected to said truss that the lowermost portion of said chain is a chord of said truss, and that the remainder of said chain is in spaced relation to and above said truss.

8. In a suspension bridge, a main-span stiffening truss, a substantially straight bottom chord on said truss throughout the span thereof, a chain connected at its midportion to the top chord of said truss and suspenders designed for tension connecting the remainder of said chain to the outer portions of said top chord, and a side-span truss continuous with said main-span truss.

9. In a suspension bridge, a main-span stilfening truss having the greatest depth lOO approximately atthe quarter points thereof, a substantially straight bottom chord on said truss throughout the span thereof, a chain directly connected to and identical with said truss at the mid-portion thereof, and suspenders designed for tension connecting the outer portions of said truss with the outer portions of said chain.

10. In a suspension bridge, a main-span stifi'ening truss comprising a substantially straight bottom chord, and a top chord having its outermost portions designed primarily for compression variably spaced .itrom said bottom chord for providing maximum truss depth at the quarter points of said truss, a pair of towers, a chain suspended from said towers, a midportion of said chain directly connected to, identical with, and forming members of, said truss between said quarter points, rigid compression members in said upper chord outside of said quarter-points and suspenders in tension only connecting the outer portions of said chain to the outer portions of said truss.

11. In suspension bridge, a main-span stiffening truss comprising a substantially straight bottom chord, and a top chord variably spaced from said bottom chord for providing maximum truss depth at the quarter points and depths at other points of the span substantially proportional to the bending moments thereat, a side-span stiffening truss continuous with said main span stiffening truss, a pair of towers, a chain suspended from said towers comprising a midpoition directly connected to, identical with, and forming part of said main-span truss between said quarter points and outer suspensory portions in spaced re lation to and, above said truss, and suspenders for hanging the outer portions of said main-span truss from said suspensory portions of said chain.

12. In a suspension bridge, a stiiiening truss, and a chain comprising a section forming part of the top chord of said truss, a second section in spaced relation to and above said truss from which the remainder of said truss is supported, and

straight backstays.

13. In a suspension bridge, a pair of towers, a stiffening truss, a chain suspended from said towers comprising a truss section forming a. chord member of said truss, a suspensory section for part of said truss, a substantially straight bottom chord throughout the span of said truss, and straight, anchored backstay sections.

14:. In a suspension bridge, towers, a stiffening truss of varying depth arranged between said towers, said depth at any point of the span being proportional to the bending moment at said point, a chain suspended from said towers and replacing part of the upper chord of said truss, and straight anchored backstays engaging said towers.

Signed at New York, in the county of New York, and State of New York, this 14th day of February, 1923.

HOLTON D. ROBINSON. DAVID B. STEINMAN. 

